When I add SRT file to "source files", it is added as "language: undetermined".
But frequently files has names as "filename.en.srt", "filename.english.srt" - so language could easily be determined.

Also sometimes subtitles are in separate folder tree, with "english", "heairing impaired", etc., somewhere in path.
So, it is possible to add whole tree of folders with SRT and autodetect all languages.